This invention relates to a ball valve element for a rotary ball valve assembly, and more particularly to a method of forming such a ball valve element.
Rotary valve assemblies are well known, such as described in WO03/087643 and shown in FIG. 1, and generally comprise a spherical or substantially spherical ball valve element 1 having a fluid-flow bore or conduit 2 therethrough. To provide pressure control of, and energy dissipation from, liquid flowing through the ball valve element, an impedance element 3 is commonly provided in the bore or conduit. The impedance element defines a typically tortuous path along which the fluid can flow, resulting in the desired pressure change.
The impedance element can be a separate device which is located in the bore or conduit of the ball valve element. However, an opening 4 to the bore or conduit, leading to the impedance element, then has to be provided either integrally formed as part of the ball valve element or as a further separate element. In either case, an external surface 5 of the ball valve element which defines the opening 4 to the impedance element must be accurately machined to match the internal dimensions, and specifically the internal radial dimensions, of a housing 6 of the rotary ball valve assembly in which the ball valve element will be rotatably located.
In the case when the impedance assembly itself partly defines an exterior surface 7 of the ball valve element, again this part of the impedance assembly must be accurately formed to fit the internal dimensions of the ball valve housing.
Accurately post-forming part of an exterior surface of a ball valve element, so that the exterior surface corresponds to the internal dimensions of the ball valve housing, is problematic and time consuming.
The present invention seeks to provide a solution to this problem.